I have a query that my certifications are connected to my present e-mail, i.e. my company's official e-mail. What if I switch to another company? How can I make sure that my certifications are still valid, and how can I transfer my certifications to a new e-mail (my next company's official e-mail)?

I have a query that my certifications are connected to my present e-mail

Your certifications so far as Oracle is concerned are connected to your OTN account. The email associated with your OTN account can be changed using the 'Your Control Panel' link in the upper-right of this screen.

You'll also want to change the email associated with your Pearson Vue account. Log in to your Pearson Vue account and use the 'Update Profile' option to change the address.